Build muscle, burn fat, all at the same time?

As an ectomorph, I have always had a hard time gaining weight. I weighed 148 lbs when I was competing in Jiu Jitsu tournments back in the day, and I only needed to cut 3 lbs to fight in the 145 lbs division. After my competition days, I had only gained up to 15 more pounds... to 160 lbs. So I was always concerned with building a bigger chest, shoulders, arms, back and legs, while burning fat around the waist.

So for a long time, I was lifting weights on Mondays, Wednesdays, and Fridays, while Tuesdays, Thursdays and Saturdays were cardio days. One would think that this is the perfect routine to build bigger muscles while working off your waistline at the same time. But I've learned that this method does not work. There is this thing which researchers call the "interference effect".

The interference effect is when you are doing two opposing types of exercise in the same period, you body will be confused and it will choose to react to one or the other. In most cases, it will choose endurance and fat loss over muscle growth. You will lose fat. Your endurance capacity will increase, your overall strength might increase also, but not your muscle size.

So, the most common way to get around this is to focus on fat loss first before trying to pack on muscle. Another way, if you have the time, is to do both in the same session. Combine an hour of lifting with an hour of cardio immediately after, three times a week. This method has been shown to be successful in multiple studies at the University of Winsconsin on athletes who wished to gain endurance and muscle size at the same time.